The present invention relates to converter circuits and, more particularly, to digital-to-analog convertors (DAC's) suited to be manufactured in monolithic integrated circuit form and operated from a single source of power supply. The prior art is replete with DAC's of the type to which the present invention is related. A typical prior art DAC includes circuitry for generating a reference current, a binary weighted R2R resistive ladder network and switching circuitry responsive to a binary digital input signal code for switching current to the ladder network thereby producing a corresponding analog output current. For instance, the MC1408, which is manufactured by Motorola, Inc., is an 8-bit DAC of this type.
Prior art DAC's have required trimmable resistors for obtaining precise values. Trimming is commonly implemented by using thin film resistors which are laser trimmed. The thin film resistors are typically implemented by putting a thin film layer over a thick field oxide layer. Laser trimming is generally restricted to thin film processing which must be implemented before packaging at a wafer probe stage. If the die is stressed during or after packaging, the value of the resistors may change. Thus, resistor trimming is not desirable.
Other prior art DAC's, in order to eliminate the need for trimmable resistors, utilize a segmented approach. In this approach, a plurality of equal weighted or equal magnitude currents are produced corresponding to a step group. An additional current step of equal magnitude is used in conjunction with a nine bit master/slave DAC to produce a step group having 512 individual current steps. As a step group is generated by stepping through each of the individual 512 current steps, an individual one of the equal weighted currents is sourced to the output of the DAC. Thus, for example, as the binary digital input signal code increases from a minimum to the first second segment or current step, 512 individual current steps are selected. Thereafter, the first current step group is held while the nine-bit DAC steps once again through these 512 levels in response to the next higher digital input signal code. The currents are summed through a summing network to the output of the DAC.
Although the aforedescribed segment DAC approach eliminates the need for trimmable resistors it does require dual power supplies for operation. Thus, it is not suited for operation in conjunction with single, low voltage power supply systems such as five (5) volt microprocessor applications.
Hence, a need exists for a single power supply, low voltage operated DAC which does not require the use of trimmable resistors for accuracy and in which the operation is inherently monotonic.